Too much of a good thing can be bad for the liver. Chen et al. find that mice with high levels of thiamine (vitamin B1) in their livers develop fatty liver disease, a metabolic disorder that affects one-third of adults in the United States. A protein called organic cation transporter 1 (OCT1) carries dietary thiamine into the liver. When the researchers deleted the Oct1 gene in mice or fed mice a diet low in thiamine, the mice did not develop the disease. OCT1 also carries the diabetes drug metformin into the liver, which might explain why metformin decreases symptoms of fatty liver disease: By competing with thiamine for OCT1, metformin reduces the amount of dietary thiamine that reaches the liver.